Optical modulator

ABSTRACT

An optical modulator restricted in a photorefractive phenomenon caused by a stray light in an optical modulator, and improved in the quenching ratio characteristics of a signal light. The optical modulator comprises a substrate consisting of a material having an electro-optic effect, an optical waveguide formed on the substrate, and a modulating electrode for allowing an electric field to work on the optical waveguide and changing the phase of light passing through the optical waveguide, characterized in that stray light removing means are provided on the surface of the substrate.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to an optical modulator provided outsideof a light source in order to modulate the light from the light source,in particular to an optical modulator restricting a photorefractivephenomenon in the optical modulator.

(2) Related Art Statement

A dense wavelength division multiplexing (DWDM) technology and highspeed communication technology have been developed for opticalcommunication systems corresponding to an increase in the demand forhigh speed, large capacity data communication systems recently.Particularly, although the modulation frequency of an optical modulatoris mostly 10 GHz, high speed modulation more than 40 GHz would also berequired from now on.

As the optical modulator which corresponds to high speed modulation, thecombination of CW (Continuous Wave) laser and the Mach-Zehnder(MZ) typeexternal optical modulator (hereinafter described as LN opticalmodulator) using the material with an electro-optic effect, such aslithium niobate(LN), have been proposed and put to practical use.

Because LN optical modulator has small wavelength dependency, it issuitable for application in DWDM type optical modulator. Also, becausethere is no modulation bandwidth limit of dielectric loss, it enablesextremely high speed modulation.

Like the optical modulator of 40 GHz, by increasing the light inputpower inputted into an LN optical modulator for the long distancetransmission, degradation of an extinction ratio, increase of an opticalloss and fluctuation of the bias point are induced. Especially when thelight input power is more than 10 mW, such problems become evident. As aresult of studies by the present inventors, they found out that themajor factor is that the stray light generated from the input part whichinputs laser light to an optical modulator and from an optical waveguidein the optical modulator, and the signal light which passes through theoptical waveguide, in particular, interfere mutually, a photorefractivephenomenon is generated, and grating is written at the optical waveguidepart by spatial overlap of stray beam and propagating beam.

Such grating written at the optical waveguide will cause degradation ofthe extinction ratio by reflecting the signal light that passes throughthe optical waveguide, in a direction opposite to the travelingdirection, or by reflecting it outward from the optical waveguide.

The photorefractive phenomenon means the phenomenon that exposure tolight varies the refractive index of an electro-optic material. Inparticular, due to the characteristic that a charge transfer isgenerated in the material by light, when optical distribution causesspatial intensity distribution of light, re-distribution of chargeoccurs corresponding to said intensity distribution of light, and thisuneven distribution of charge varies an internal electric fieldtopically. Because the internal electric field varies the refractiveindex of the material, refractive index distribution of the materialthat corresponds to the intensity distribution of light is formedresultantly.

Further, the photorefractive phenomenon has the characteristic that therefractive index changes little by little when being continuouslyexposed to light, and a light scattering gets stronger and stronger astime goes by. Therefore, in drive of an optical modulator for manyhours, the deterioration of the optical modulator characteristics,especially degradation of the extinction ratio, increase of the opticalloss, etc. becomes prominent.

The present invention intends to solve the above problems, to restrictthe photorefractive phenomenon caused by a stray light in the opticalmodulator, and to provide the optical modulator which improves thecharacteristics relevant to the extinction ratio or optical loss of asignal light.

Particularly, the photorefractive phenomenon tends to occur for theoptical modulator having a Mach-Zehnder type optical waveguide sincethere are many opportunities of interference with the stray light due toescaping light from a branching point of the branching optical waveguideand longer optical waveguide active part that allows phase modulation towork on the signal light passing through the optical waveguide. Further,for the optical modulator having so called dual electrode constructionwhich drive controls several optical waveguide active parts by anindependent modulating electrode separately, it is necessary to keepenough distance between modulating electrodes for avoiding cross talkbetween said modulating electrodes. This makes the length of thewaveguide after the branching point of the branching optical waveguidelonger, which increases the chances of interfering with the stray lightand the photorefractive phenomenon tends to occur as a result.

SUMMARY OF THE INVENTION

In order to solve the above described problems, the invention related toclaim 1 provides an optical modulator comprising a substrate consistingof a material having an electro-optic effect, an optical waveguideformed on said substrate, and a modulating electrode for allowing anelectric field to work on said optical waveguide, and changing the phaseof light passing through said optical waveguide, wherein stray lightrejection means are provided on the surface of said substrate.

In accordance with the invention related to claim 1, the stray lightrejection means avoids a diffusion of the stray light, in particular,that scatters parallel to the surface of the substrate, out of the straylight escaping from the optical waveguide formed on the substrate of theoptical modulator. Thus, the stray light doesn't enter another opticalwaveguide in the substrate, the stray light and a signal light passingthrough said optical waveguide don't interfere mutually, andaccordingly, no interference grating is generated. This provides thepossibility of restricting a photorefractive phenomenon.

In addition, the invention related to claim 2 provides the opticalmodulator according to claim 1, wherein said stray light rejection meanscomprises a stray light rejection groove, at least one part of which isformed adjacent to said optical waveguide.

In accordance with the invention related to claim 2, for composing thestray light rejection means of the groove formed on the substrate, knownfine processing technologies such as etching, laser beam machining, andcutting works like sand blast can be applied, with which the stray lightrejection means can easily formed. Further, because such stray lightrejection groove is formed adjacent to the optical waveguide, it ispossible, for example, to reject the stray light exiting from theoptical waveguide before a diffusion, for the optical waveguide wherethe stray light exits, and to forestall the interference of the signallight passing through the optical waveguide and the stray light, for theoptical waveguide which the stray light enters.

In addition, the invention related to claim 3 provides the opticalmodulator according to claim 2, wherein the distance between said straylight rejection groove and said optical waveguide is 10 to 100 μm atclosest.

In accordance with the invention related to claim 3, by making theclosest distance between the stray light rejection groove and theoptical waveguide 10 μm or more, the stray light rejection groove can beformed with good accuracy without damaging the optical waveguide.Especially when the groove is formed by a mechanical processing method,the optical waveguide (or the substrate portion where the opticalwaveguide is formed) does not have distortion caused by mechanicalprocessing. Therefore, it is possible to maintain the characteristics ofthe optical waveguide stably. Also, by making the closest distance lessthan 100 μm, it is possible to reject the diffusion of the stray lightfrom the optical waveguide, or the entrance of the stray light to theoptical waveguide effectively, and to restrict the photorefractivephenomenon.

In addition, the invention related to claim 4 provides the opticalmodulator according to any of claims 2 and 3, wherein the depth of saidstray light rejection groove is almost the same or more than that ofsaid optical waveguide.

In accordance with the invention related to claim 4, because the depthof the stray light rejection groove is almost the same or more than thatof the optical waveguide, it provides the possibility of rejecting thestray light effectively in case of the diffusion of the stray light fromthe deepest part of the optical waveguide, or the entrance of the straylight to the deepest part of the optical waveguide.

“Almost the same” means the same depth or the depth where the effect,which is substantially no way inferior to the effect obtained from thesame depth, can be obtained.

In addition, the invention related to claim 5 provides the opticalmodulator according to any of claims 2 to 4, wherein said stray lightrejection groove is filled with a light absorber material.

In accordance with the invention related to claim 5, due to the lightabsorber material filled in the stray light rejection groove, it ispossible to obstruct the course of the stray light by said grooveitself, as well as to prevent a scattering of the stray light on thesurface of the groove with the light absorber material. As a result, theeffect of rejecting the stray light improves further.

In addition, the invention related to claim 6 provides the opticalmodulator according to any of claims 1 to 5, wherein said opticalwaveguide comprises a branching optical waveguide, and at least one partof stray light rejection means is provided adjacent to said branchingoptical waveguide.

In accordance with the invention related to claim 6, for the opticalmodulator having the branching optical waveguide like a Mach-Zehndertype optical modulator, the stray light rejection means providedadjacent to the branching optical waveguide enables not only thediffusion of an escaping light, the cause of the stray light, from abranching point of the branching optical waveguide to be prevented, butalso the scattering light at an input part inputting laser light fromthe outside of the optical modulator to be restricted not to enter thebranching part of the branching optical waveguide and generate aninterference grating.

In addition, the invention related to claim 7 provides the opticalmodulator according to any of claims 1 to 5, wherein at least one partof said stray light rejection means is provided between said opticalwaveguide that the electric field of the modulating electrode works onand the side face of the substrate that is close to said opticalwaveguide.

By providing the stray light rejection means between the opticalwaveguide that the electric field of the modulating electrode works onand the side face of the substrate that is close to said opticalwaveguide as in the invention related to claim 7, especially when anactive part (hereinafter described as “optical waveguide active part”)of the optical modulator that allows phase modulation to work on thesignal light is relatively long compared with the entire opticalwaveguide as the optical modulator having the Mach-Zehnder type opticalwaveguide, it is possible to prevent the stray light from entering saidoptical waveguide active part effectively.

In addition, the invention related to claim 8 provides an opticalmodulator comprising a substrate consisting of a material having anelectro-optic effect, an optical waveguide formed on said substrate, anda modulating electrode for allowing an electric field to work on saidoptical waveguide, and changing the phase of light passing through saidoptical waveguide, wherein a low refractive index area with therefractive index lower than that of said substrate is provided at oneportion of the adjacent spaces comprising at least the lower portion andthe side portion of said optical waveguide in order to prevent a straylight from entering the optical waveguide.

In accordance with the invention related to claim 8, for the straylight, in particular, that scatters in the direction of the reverse faceof the substrate, out of the stray light escaping from the opticalwaveguide formed on the substrate of the optical modulator, the lowrefractive index area prevents the stray light from reentering theoptical waveguide, the stray light and the signal light which passesthrough the optical waveguide don't interfere mutually, and nointerference grating is generated. As a result, it is made possible torestrict the photorefractive phenomenon.

This is because providing the low refractive index area with therefractive index lower than that of the substrate enables the straylight entering from the material side of the substrate to be reflectedat the surface of the low refractive index area (a boundary surfacebetween a substrate material in the substrate and a material forming thelow refractive index area). In particular, it is possible to leak outthe escaping light from the optical waveguide, to reject only the straylight effectively which is to enter the low refractive index area fromthe outside of the low refractive index area (opposite to the side wherethe optical waveguide is formed, the boundary of which is the lowrefractive index area), and thereby to prevent the stray light fromentering the optical waveguide. In order to prevent the stray light fromentering from the reverse face side of the substrate more effectively,it is preferable to form the low refractive index area at the lowerportion side or side portion side of the optical waveguide.

In addition, the invention related to claim 9 provides the opticalmodulator according to claim 8, wherein said low refractive index areahas thickness longer than the depth of said optical waveguide in thethickness direction of the substrate from the surface of said substrate,and the refractive index between the deepest part of said low refractiveindex area and the reverse face of said substrate is higher than that ofsaid low refractive index area.

In accordance with the invention related to claim 9, because thethickness of the low refractive index area has thickness longer than thedepth of said optical waveguide in the thickness direction of thesubstrate from the surface of the substrate, it presents the possibilityof preventing the stray light which is to enter the deepest part of theoptical waveguide from entering. The possible range of avoiding thestray light with said low refractive index area out of the incidenceangle of the stray light entering the optical waveguide depends on therefractive index and location of the low refractive index area.Particularly, it is effective to locate the low refractive index area atthe lower portion side. However, it is more preferable to surround theoptical waveguide with the low refractive index area entirely. Thisenables preventing of the stray light entering the optical waveguideeffectively.

Further, by making the whole substrate from its surface to certain depththe low refractive index area, when the low refractive index are isformed, it is possible to form the low refractive index area more easilywithout making the pattern formation in accordance with the opticalwaveguide by photolithography etc.

Also, by making the refractive index between the deepest part of thelower refractive index area and the reverse face of the substrate higherthan that of said low refractive index area, it is possible to avoid thestray light at the surface of the substrate, which is reflected at thereverse face of the substrate, or to prevent it from entering the lowrefractive index area. As a result, it is made possible to prevent thestray light from entering the optical waveguide effectively. Further,for the distribution of the refractive index between the deepest part ofthe low refractive index area and the reverse face of the substrate, bymaking the refractive index high in a stable condition, or making itgrowing into an high refractive index, it is possible to reject thestray light reflected at the reverse face of the substrate moreeffectively.

In addition, the invention related to claim 10 provides the opticalmodulator according to any of claims 8 and 9, wherein said lowrefractive index area is formed by diffusion of a low refractive indexmaterial with the refractive index lower than that of said substrateover said substrate.

In accordance with the invention related to claim 10, a refractive indexadjusting means by ionic diffusion, which is frequently used in theproduction process of the optical modulator, is available. Withoutadding any special device or complicated process, but only by setting adiffusion process for forming the low refractive index area in theexisting production process of the optical modulator, it is possible toproduce the optical modulator having the low refractive index areaeasily.

In addition, the invention related to claim 11 provides the opticalmodulator according to any of claims 8 to 10, wherein said lowrefractive index area comprises MgO or ZnO as the low refractive indexmaterial.

In accordance with the invention related to claim 11, in adjusting therefractive index of the substrate by the ionic diffusion, morehomogeneous low refractive index area can be formed by applying MgO orZnO, diffusion of which is easy to adjust. In particular, it can bepreferably applied to the low refractive index adjustment of an LNoptical modulator, which is predominant currently.

In addition, the invention related to claim 12 provides an opticalmodulator comprising a substrate consisting of a material having anelectro-optic effect, an optical waveguide formed on said substrate, anda modulating electrode for allowing an electric field to work on saidoptical waveguide, and changing the phase of light passing through saidoptical waveguide, wherein a high refractive index area with therefractive index higher than that of said substrate is provided on thereverse face or side face of said substrate.

The invention related to claim 12 enables reflecting of the stray light,which was reflected at the reverse face or side face of the substrate,at the boundary surface of the substrate material in the substrate and amaterial forming the high refractive index area, and thereby restrictingof the stray light moving toward the surface of the substrate where theoptical waveguide is formed.

In addition, the invention related to claim 13 provides the opticalmodulator according to any of claims 1 to 12, wherein antireflectiontreatment is given on the reverse face or side face of said substrate.

The invention related to claim 13 makes it possible to prevent the straylight from being reflected at the reverse face or side face of thesubstrate, and to restrict the stray light not to enter the opticalwaveguide.

In addition, the invention related to claim 14 provides the opticalmodulator according to claims 1 to 13, wherein the frequency ofmodulation drive is more than 40 GHz.

In accordance with the invention related to claim 14, in driving theoptical modulator especially with the frequency of modulation drive morethan 40 GHz where the influence of the photorefractive phenomenonbecomes significant, it is possible to avoid the degradation of asuperior extinction ratio or increase of an optical loss by rejectingthe stray light and restricting mutual interference of the signal lightpassing through the optical waveguide and the stray light.

In addition, the invention related to claim 15 provides the opticalmodulator according to any of claims 1 to 14, wherein the input power ofthe light that is inputted into said optical modulator is more than 10mW.

In accordance with the invention related to claim 15, in putting thelight having an optical input power more than 10 mW especially, wherethe effect of the photorefractive phenomenon becomes significant, to theoptical waveguide, it is possible to avoid the degradation of thesuperior extinction ratio or increase of the optical loss by rejectingthe stray light and restricting mutual interference of the signal lightpassing through the optical waveguide and the stray light.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the generation status of the stray light inan existing optical modulator.

FIG. 2 is a schematic diagram showing the optical modulator providedwith the stray light rejection means of the preset invention.

FIG. 3 is a diagram showing the positional relation of the opticalwaveguide and the stray light rejection means.

FIG. 4 is a diagram showing the generation status of the stray lightpassing through to the thickness direction of the substrate in theexisting optical modulator.

FIG. 5 is a diagram showing the status where the low refractive indexarea is formed only around the optical waveguide.

FIG. 6 is a diagram showing the status where the substrate of theoptical modulator, to certain thickness, is made the low refractiveindex area.

FIG. 7 is a diagram showing the status where the high refractive indexarea is formed on the reverse face and side face of the opticalmodulator.

DETAILED DESCRIPTION OF THE INVENTION

In the following, the preferred embodiments of the present invention areexplained in detail.

The substrate which configures an optical modulator is made of amaterial having an electro-optic affect, such as lithium niobate(LiNbO₃; hereinafter referred to as LN), lithium tantalite (LiTaO₃), orPLZT (lead lanthanum zirconate titanate). In particular, it ispreferable to use a LiNbO₃ crystal, a LiTaO₃ crystal, or a solidsolution crystal made of LiNbO₃ and LiTaO₃ due to the fact that anoptical waveguide device can be easily formed of any of these crystalswhich have a large anisotropy. The present invention embodimentprimarily refers to an example using lithium niobate (LN).

A method for forming an optical waveguide by thermal diffusion of Ti inan LN substrate, and subsequently forming an electrode directly on theLN substrate without providing a buffer layer over a portion or theentirety of the substrate, and a method for providing a buffer layer,such as SiO₂ which is dielectric, on an LN substrate in order to reducethe propagation loss of light in the optical waveguide and forming amodulating electrode and a grounding electrode having thickness ofseveral tens of μm on top of the buffer layer according to the formationof a Ti•Au electrode pattern, and according to a gold plating method orthe like, are cited as methods for manufacturing an optical modulator.

In general, a plurality of optical modulators are fabricated on one LNwafer, which is cut into individual optical modulators at the last stageand thereby, optical modulators are manufactured.

FIG. 1 is a diagram showing a skeletal form of an existing LN opticalmodulator.

Numeral 1 is the LN substrate, and the waveguide is formed on thesurface of the substrate by internally diffusing Ti etc. as abovedescribed. 2 is an input waveguide, into which the light from a CW lasersource, which is not shown in the diagram, is guided, and which isconnected to a fiber 3 having a polarization holding feature.

The light passing through the waveguide 2 is equally divided at a 3 dBbranching optical waveguide 4, which is a first branching opticalwaveguide, and respectively put into an optical waveguide active part 5that configures the arm of a Mach-Zehnder (MZ) type optical waveguide.

A modulating electrode and a grounding electrode, which are not shown inthe diagram, are located adjacent to said optical waveguide active part5. The light passing through the optical waveguide active part inaccordance with the signal impressed to the modulating electrodereceives phase modulation. After the phase modulation, each guided waveis joined together at a second branching optical waveguide 6, andthereby generates a signal light which is strongly modulated by mutualinterference.

The signal light passes through an output waveguide 7 and then, is takenoutside of a module from an output fiber 8.

For the existing optical modulator, as shown in FIG. 1, stray lights aand b escape from the junction of the fiber 3 and the input waveguide 2of the optical modulator, and further, stray lights c and d escape fromthe branching point of the first branching optical waveguide 4. Eachstray light enters the first optical waveguide 4, the optical waveguideactive part 5, the second branching optical waveguide 6, etc.,interferes with the light passing through said optical waveguide andgenerates an interference grating as a result. This interference gratinggenerates a photorefractive phenomenon, and thereby causes degradationof an extinction ratio of the signal light. Also, in the input waveguide2 and the output waveguide 7, the interference grating leads todegradation of an extinction ratio since the light passing through theoptical waveguide is likewise scattered.

In order to eliminate such effect of the stray light, the presentinvention places stray light rejection means 11 to 22 adjacent to theoptical waveguide such that the mutual interference of the stray lightand the light passing through the optical waveguide is restricted asshown in FIG. 2. In particular, each alignment and shape are configuredsuch that the stray light e is rejected with the means 11, the straylights f and g, which can not be rejected with the means 11 (or in casethere is no means 11), are rejected with the means 13 and 14, the straylight h is rejected with the means 12 and 17, the stray light i isrejected with the means 18, and the stray lights j and k are rejectedwith the means 18, 19 and 20.

The stray light rejection means prevents the stray light from reachingthe optical waveguide by forming a groove, depth of which (about 50 μm)is same as that of the optical waveguide, on the surface of thesubstrate 1 and applying a scattering of the light at the wall surfaceof the groove.

As the method for forming the groove, there is one easy method where asubstrate material is partly removed by laser beam machining and therebythe groove is formed. Besides, well-known processing techniques in therelevant field such as a chemical processing method where the substrateis grooved by etching, or a chemical cutting method of sand blast, etc.can be also applied.

As the method for strengthening the features of stray light rejection inthe above groove, the stray light which passes through said groove isblocked by filing a light absorber material such as carbon black intosaid groove.

Also, in general, as shown in FIG. 3, the closer the optical waveguide(the input waveguide 2 in the diagram) and the stray light rejectionmeans (the grooves 11 and 12) are placed to each other, the higher therejecting effect becomes. However, there is a technical limit such thatthe optical waveguide is not damaged but can be formed with accuracy inthe production process and it is also necessary to consider reduction ofa distortion of the optical waveguide (or the substrate portion wherethe optical waveguide is formed) in the mechanical process such as acutting process. The line width of the optical waveguide is normallyabout 7 μm, and the distance between the edge boundary of the straylight rejection means and the optical waveguide is preferably longerthan 10 μm as 15 μm in FIG. 3.

On the other hand, if the above distance is longer than 100 μm, thescattering of the stray light from the optical waveguide and incidenceof the stray light to the optical waveguide can not be effectivelycontrolled and therefore, it might not be possible to obtain thedesirable stray light rejection effect.

Although the width of the stray light rejection means is set to be 80 μmin FIG. 3, any width is acceptable as long as the groove is formedtherein, basically. The stray light rejection means should be formedtaking various points into consideration as described below.

The alignment and shape of the stray light rejection means such asgroove, though various types can be suggested, are decided based mainlyon the following points.

1. Preventing Primarily the Scattering of the Stray Light

-   (1) One which directly blocks the stray light from the input end of    the optical modulator (11, 12, 13 to 16 in FIG. 1)-   (2) One which directly blocks the stray light from the branching    point of the first branching optical waveguide of the optical    modulator (18, 19, 20 in FIG. 1)-   (3) One which blocks the stray light reflecting from the side face    of the substrate of the optical modulator (13 to 16, 17 in FIG. 1)

Besides, an escaping light could be generated in the second branchingoptical waveguide or a curve portion of the optical waveguide. It isalso necessary to deal with these situations if required.

2. Preventing the Stray Light from Entering the Optical Waveguide

One which places the stray light rejection means adjacent to thesurrounding area of the optical waveguide where the stray light shouldbe prevented from entering (17, 19, 20 in FIG. 1)

3. Consideration of the Shape and Lead Wire of the Modulating Electrodeand the Grounding Electrode

It is also possible to adjust the alignment and shape of the stray lightrejection means taking into consideration the shape and lead wire of themodulating electrode and the grounding electrode as 11, 12, 13 to 16 and17 in FIG. 1.

The second embodiment of the present invention is explained in thefollowing.

As shown in FIG. 4, there exist stray lights 1 and m having a vectorcomponent in the thickness direction of the substrate for the straylight of the optical modulator, as well as the stray light in parallelwith the surface of the substrate.

The stray light like the stray lights 1 and m that moves in thethickness direction of the substrate reflects at a base 30 or the sideface of the substrate, enters the optical waveguide, and possiblyinterferes with the light passing through the optical waveguide.

In order to reject such stray light, as shown in FIG. 5, a lowrefractive index area 40 is formed such that it surrounds the opticalwaveguide.

By making the refractive index of the low refractive index area lowerthan that of the substrate, stray lights o and h that are releasedoutside of the low refractive index area are reflected at the boundarysurface of the substrate and the low refractive index area, and arethereby prevented from entering the optical waveguide that is placedinside of the low refractive index area.

As the alignment of the low refractive index area against the opticalwaveguide, besides the one where the low refractive index area surroundsthe entire optical waveguide as shown in FIG. 5, it is possible toconfigure it to reject only the unnecessary stray lights by selectivelyplacing it on the lower portion side or side portion side of the opticalwaveguide. Preferably, the low refractive index area is formed in theadjacent spaces of the optical waveguide comprising the lower portionside and side portion side of the optical waveguide.

In addition, FIG. 5( b) shows a cross-section shape at a dashed line Ain FIG. 5( a).

As the other alignment of the low refractive index area, as shown inFIG. 6, it is possible to form the low refractive index area over theentire surface of the substrate to certain depth wherein the opticalwaveguide is comprised. Here, in order to form the low refractive indexarea in accordance with the shape of the optical waveguide as in FIG. 5,it is necessary to separately prepare a photomask for forming the lowrefractive index area (however, it is also possible to use at the sametime mask pattern for the optical waveguide as described in thefollowing), and therefore, the production process gets complicated andexpensive somewhat. On the other hand, when the low refractive indexarea is formed over the entire surface of the substrate as shown in FIG.6, it is possible to skip such process.

As the method for forming the low refractive index area, materials suchas MgO, ZnO, Na₂O, Li₂O, B₂O₃, or K₂O, having lower refractive indexthan that of an LN substrate material are diffused over said substrate.In addition, Fe₂O₃, NiO, or Cu₂O, are also impurities which decrease therefractive index. However, they are not preferable since they improveoptical loss sensitivity of an LN crystal.

For example, a thermal diffusion method is used as the diffusion method.In particular, the low refractive index material is deposited around anoptical waveguide forming area to given thickness by using the maskpattern that is applied in forming the optical waveguide, the substrateis heated to given temperature, and the low refractive index material isthermally diffused in the substrate.

Such thermal diffusion can be conducted before or after the process forforming the optical waveguide. However, it is preferable to conduct itbefore the process for forming the optical waveguide such that theoptical waveguide that has been already formed do not suffer the badeffect by the thermal diffusion processing of the low refractive indexmaterial.

In addition, the above described mask pattern is not required in formingthe low refractive index area as shown in FIG. 6.

As for the thickness of the low refractive index area, when thethickness is more than the depth of the optical waveguide from thesurface of the substrate to the thickness direction of the substrate, itis possible to prevent the stray light that is to enter toward thedeepest part of the optical waveguide from entering.

Furthermore, the possible range of avoiding the incidence angle withsaid low refractive index area out of that of the stray light enteringthe optical waveguide depends on the refractive index and alignment ofthe low refractive index area. Particularly, it is effective to placethe low refractive area at the lower portion side. However, it ispreferable to surround the optical waveguide by the low refractive indexarea entirely as shown in FIG. 5 and FIG. 6. This enables preventing ofthe stray light entering the optical waveguide effectively.

Also, by making the refractive index between the deepest part of the lowrefractive index area and the reverse face of the substrate higher thanthat of said low refractive index area, it is possible to prevent thestray light reflected at the reverse face of the substrate, or toprevent the stray light from entering the low refractive index area. Theincidence of the stray light to the optical waveguide can be effectivelyrestricted as a result. FIG. 5 and FIG. 6 show the one with therefractive index distribution at a constant state between the deepestpart of the low refractive index area and the reverse face of thesubstrate.

In addition, it is possible to form said increasing state by doping Ti,Ta, Fe, Ag, La, and Y, which are materials having the high refractiveindex, from the reverse face of the substrate into the substrate.

As for the space between the optical waveguide and the low refractiveindex area, it is preferable to configure it such that said space doesnot exist by placing the optical waveguide and the low refractive indexarea adjacent to each other. This is because the stray light escapingfrom the optical waveguide is reflected at the boundary surface on theside of the optical waveguide of the low refractive index area andthereby generates a problem that the stray light is trapped in the spacecomprising the optical waveguide in case the optical waveguide and thelow refractive index area are formed distantly.

Subsequently, the third embodiment is explained.

As shown in FIG. 7, a high refractive index area 42 is formed on thereverse face (base) or side face of the substrate. As the method forforming the high refractive index area, doping a material having saidhigh refractive index into the substrate by thermal diffusion etc canform the high refractive index area.

The high refractive index area enables the stray light reflecting at thereverse face or side face of the substrate to be trapped in the highrefractive index area, and therefore to be prevented from moving towardthe optical waveguide again.

Further, in order to reject stray light reflection from the base or sideface of the substrate of the optical modulator more effectively,antireflection treatment, for example coating these faces with anoptical absorber material such as carbon black, or an antireflectioncoating, can be given.

Also, combining above described various embodiments if necessary canimprove the effectiveness of stray light rejection further.

As the embodiments of the present invention are described above, thepresent invention is not limited to the scope of the above embodiments,but comprises the ones where technical configuration is substituted by atechnology well know in the art.

As described above, according to the optical modulator of the presentinvention, because the escaping light from the optical waveguide isprevented from diffusing and the stray light is restricted not to enterthe optical waveguide, the photorefractive phenomenon caused by thestray light in the optical modulator can be restricted and it ispossible to provide the optical modulator which improves characteristicsrelating to extinction ratio or optical loss of the signal light.

In particular, this invention is able to restrict the photorefractivephenomenon, the causing the degradation of extinction ratio etc. whichappears prominently when the optical modulator with the Mach-Zehndertype optical waveguide has more than 40 GHz of drive or more than 10 mWof optical input power.

1. An optical modulator comprising a substrate comprised of a materialhaving an electro-optic effect, an optical waveguide formed on saidsubstrate, and a modulating electrode for allowing an electric field towork on said optical waveguide, and changing a phase of light passingthrough said optical waveguide, wherein said optical waveguide is aMach-Zehnder type optical waveguide, stray light rejection means isprovided on a surface of said substrate and at least one part of saidstray light projection means is disposed between the optical waveguideactive part, where the electric field of said modulating electrode workson the optical waveguide, and the substrates side face near said opticalwaveguide active part.
 2. The optical modulator according to claim 1,wherein said stray light rejection means comprises a stray lightrejection groove, which is formed on the surface of said substrate, andat least one part of which is formed adjacent to said optical waveguide.3. The optical modulator according to claim 2, wherein a distancebetween said stray light rejection groove and said optical waveguide is10 to 100 μm at closest.
 4. The optical modulator according to claim 2,wherein depth of said stray light rejection groove is almost the same asor is more than depth of said optical waveguide.
 5. The opticalmodulator according to claim 2, wherein said stray light rejectiongroove is filled with a light absorbing material.
 6. An opticalmodulator comprising a substrate comprised of a material having anelectro-optic effect, an optical waveguide formed on said substrate, anda modulating electrode for allowing an electric field to work on saidoptical waveguide, and changing a phase of light passing through saidoptical waveguide, wherein said optical waveguide is a Mach-Zehnder typeoptical waveguide, and in order to prevent stray light from entering theoptical waveguide, a low refractive index area with a refractive indexlower than that of said substrate is provided at surrounding area of theoptical waveguide including an under portion and a side portion of theoptical waveguide, which comprises at least the optical waveguide activepart where the electric field of said modulating electrode works on theoptical waveguide.
 7. The optical modulator according to claim 6,wherein said low refractive index area has a thickness greater than adepth of said optical waveguide in a thickness direction of thesubstrate from a surface of said substrate, and a refractive indexbetween a deepest part of said low refractive index area and a reverseface of said substrate is higher than the refractive index of said lowrefractive index area.
 8. The optical modulator according to claim 6,wherein said low refractive index area is formed by diffusion of a lowrefractive index material having a refractive index lower than that ofsaid substrate, over said substrate.
 9. The optical modulator accordingto claim 8, wherein said low refractive index area comprises MgO or ZnOas the low refractive index material.
 10. The optical modulatoraccording to claim 1, wherein antireflection treatment is given on areverse face or a side face of said substrate.
 11. The optical modulatoraccording to claim 1, wherein the frequency of modulation drive is morethan 40 GHz.
 12. The optical modulator according to claim 1, whereininput power of light input into said optical waveguide element is morethan 10 mW.
 13. The optical modulator according to claim 3, whereindepth of said stray light rejection groove is almost the same as or ismore than depth of said optical waveguide.
 14. The optical modulatoraccording to claim 3, wherein said stray light rejection groove isfilled with a light absorbing material.
 15. The optical modulatoraccording to claim 6, wherein antireflection treatment is given on areverse face or a side face of said substrate.
 16. The optical modulatoraccording to claim 6, wherein the frequency of modulation drive is morethan 40 GHz.
 17. The optical modulator according to claim 6, whereininput power of light input into said optical waveguide element is morethan 10 mW.